Measurement of small signal magnetic fields with SQUID-based magnetometers is known. As used herein, the term small signal magnetic fields is intended to comprise magnetic field strengths such as those encountered in biomagnetometry, geophysics, SQUID-based non-destructive evaluation (SQUID NDE), etc.
It is also known to form gradiometers by either combining a pair of several pairs of magnetometers, as shown in U.S. Pat. No. 5,122,744 to Koch, or to form gradiometers which employ a pair, or pairs, of sensor coils of opposite sense as shown in U.S. Pat. No. 5,311,129 to Ludwig et al. A discussion of counter-wound gradiometers is presented in "Spatial Discrimination in SQUID Gradiometers and 3rd Order of Gradiometer Performance", J. Vrba, A. A. Fife and M. B. Burbank, Canadian Journal of Physics, 60, 1982, pp. 1060-1073, the contents of which are incorporated herein by reference.
Generally, as discussed in the above-mentioned Vrba et al. reference, gradiometers are preferred over magnetometers (as used herein, the terms magnetometer and zero order gradiometer are intended to be synonymous and references to gradiometers may also encompass magnetometers, depending upon the context of the reference, as will be apparent to those of skill in the art) as they provide improved spatial filtering of magnetic fields. In other words, non-zero order gradiometers can maintain high sensitivity to magnetic sources located relatively close to the gradiometer sensor coils and a sharply decreased sensitivity (improved rejection) to magnetic sources spaced relatively distant from the gradiometer sensor coils. Further, generally a higher order gradiometer will exhibit better rejection than a gradiometer of a lower order. Such spatial filtering is often advantageous and is particularly advantageous if the gradiometer is to be used in an unshielded or minimally shielded environment.
Due to various factors, such as physical and size constraints, manufacturability considerations and gradiometer balancing concerns, it is not generally practical to construct multi sensor systems which employ hardware second or higher order gradiometers sa sensors. Accordingly, multi sensor systems with hardware higher order gradiometer sensors have heretofore been commercially impractical.
It is desired therefore to have a method or system which provides measurements substantially equivalent to those which would be obtained with second order or higher gradiometers sensors without requiring the use of such sensors.